Ensure InlineCasts does not inline complex Expressions (#2130) (#2144)

Previously, InlineCasts could inline complex (ie. non-cast) Expressions
into other complex Expressions. Now it will only inline so long as there
no more than 1 complex Expression in the current nested Expression.

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
(cherry picked from commit b274b319

)

Co-authored-by: Jack Koenig <koenig@sifive.com>